Threads mistakenly attach to main thread's span

The first time we call sentry-related functions in a thread, it attaches to whichever *Span* the main thread is currently attached to.
This causes very wrong sub-spanning (which I just spend 6 hours investigating).
Attaching to sentry client, DSN, parameters..., etc seems correct, but span definitely not.

I feel like maybe the *Scope* should be a property that's not inherited from whichever scope the main thread happens to be in at that moment, because if there is a sub-thread it's very possible that the main thread is doing unrelated work at that moment, and that unrelated work may very well involve its own spans and data, so it seems that it's not correct to attach to those.

Related issues: [RUST-25](https://linear.app/getsentry/issue/RUST-25/provide-examples-for-async-use-of-hub)
Related writeup: [https://swatinem.de/blog/log-contexts/]()